Go with the 5/8-24 and be happy. I have threaded several barrels of varying bores and saw no difference in performance that I could relate to bore size. Performance always seemed to be improved by the brake for me.
I have had a couple of 45cal barrels (450BM and 458WM) that were both threaded 5/8-24 with no ill effects and that's about as thin a barrel sidewall as you will see. Both shoot fine.
If you feel better with 3/4-24 then go with it and just plan that if you sell it, let the brake go to0 for an easier sale, but 5/8-24 works just fine.
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